The present invention relates to fiber optic gyroscopes and particularly to a constant accuracy, high dynamic range fiber optic gyroscope.
The Sagnac phase shift between two counterpropagating beams is the basis for all optical gyroscopes, although it is detected in a variety of ways. In the ring laser gyroscope and various "closed loop" optical gyroscopes, the scale factor (e.g. counts per unit rotation rate) is fixed by the area of the optical medium, whereas the phase reading fiber optic gyroscope (FOG) has a scale factor which increases with increasing length of optical fiber. Thus, development of low-loss fibers holds the possibility of extremely sensitive gyroscope operation, and has made the FOG a competitive optical gyroscope. Although most of the first decade of FOG development has concentrated on improving sensitivity, attention is now turning to other difficulties associated with the FOG.
Principal among the difficulties associated with the FOG are the "scale factor" and the "dynamic range" problems. The scale factor problem refers to the fact that the electrical output of the fiber optic gyroscope is not always the same for a given input rotation. The dynamic range problem refers to the fact that there are limitations to the upper rotation rate (and hence the ratio of upper rate to minimum detectable rate). The "scale factor" being discussed here is not the optical scale factor relating the Sagnac phase shift .phi..sub.s to the rotation rate .OMEGA., but an "electrical" scale factor which arises from converting the optical phase shift into an electrical signal. Such scale factor and dynamic range difficulties will be explained more fully by now referring to the conventional or "minimum configuration" fiber optic gyroscope (FOG) shown in FIG. 1 (and described by S. Ezekiel, et al. in their article, Fiber Optic Rotation Sensors, Tutorial Review, in Fiber Optic Rotation Sensors, published by Springer, NY (1982)).
The fiber optic gyroscope of FIG. 1 includes a first directional coupler 13 having input ports 15 and 17 and output ports 19 and 21, a second directional coupler 23 having input ports 25 and 27 and output ports 29 and 31, and a fiber optic coil 33 having opposite ends 35 and 37 respectively coupled to the output ports 29 and 31 of the coupler 23. The fiber optic coil 33 comprises N turns wound on a plan area A. In a typical application the fiber optic gyroscope of FIG. 1 is mounted on a rotating platform (not shown) with the axis of symmetry of the coil 33 parallel to the axis of rotation of the platform so that the rotation rate .OMEGA. of the platform can be sensed.
Light from an optical source 39, such as a laser or super-luminescent diode, propagates through the coupler 13 to the coupler 23, which splits the light into two substantially equal beams I.sub.1 and I.sub.2. The beams I.sub.1 and I.sub.2 respectively enter the opposite ends 35 and 37 of the fiber optic coil 33, with beam I.sub.1 propagating through the coil 33 in a clockwise direction and beam I.sub.2 propagating through the coil 33 in a counter-clockwise direction. After these two counter-propagating light beams I.sub.1 and I.sub.2 have traversed the coil 33, they reenter the coupler 23 through its output ports 31 and 29 and, upon recombining in the coupler 23, interfere with each other. This light interference is related to the rate of rotation .OMEGA. of the coil 33. For example, if the gyroscope is rotating at a rate .OMEGA., the beams I.sub.1 and I.sub.2 undergo a non-reciprocal Sagnac phase shift, .phi..sub.s, where ##EQU1## for a source 39 of wavelength .lambda., where c is the velocity of light.
The combined interfering beams combine in coupler 23 and the resultant beam propagates into the coupler 13. A portion of that resultant beam is directed by the coupler 13 to a photodetector 41, where the intensity EQU I=I.sub.o (1+cos .phi..sub.tot) (2)
is detected and amplified by an amplifier 43. The intensity I represents the standard interferometric "fringe" pattern for the total non-reciprocal phase shift .phi..sub.tot, where I.sub.o is the peak ac excursion.
If nothing else perturbs the fiber optic coil 33, .phi..sub.tot =.phi..sub.s, and in principle this signal could be detected to determine the rotation rate. However, a practical problem is that the sensitivity of the gyroscope, .apprxeq..DELTA.I/.DELTA..phi..sub.s, goes to zero at low rotation rates. To avoid this, a time varying phase shift or modulation is usually applied, often by winding a part of the coil 33 around a piezoelectric transducer (PZT) 45. When the PZT 45 is driven sinusoidally at a frequency .omega. and drive level or amplitude A by an oscillator or PZT driver 47, a non-reciprocal phase shift is added to the Sagnac phase shift, .phi..sub.s : EQU .phi..sub.tot =.phi..sub.s +.eta.cos .omega.t. (3)
where .eta.=2 sin .omega..tau./2 and .tau. is the transit time of the coil 33. As a result of the non-linear phase applied and the non-linear interferometer response of Equation (2), the output of the amplifier 43 contains an abundance of harmonics of the PZT drive frequency .omega. and the ideal voltage output of the amplifier 43 is given by: EQU V=I.sub.o {(1+J.sub.o (.eta.)cos.phi..sub.s)+.alpha..sub.1 J.sub.1 (.eta.)sin.phi..sub.s cos.omega.t+.alpha..sub.2 J.sub.2 (.eta.)cos.phi..sub.s cos 2.omega.t+ . . .} (4)
where the .alpha..sub.i 's depend on electrical bandwidth and so forth, and the J.sub.i 's are Bessel functions.
The above voltage output of the amplifier 43, as shown in equation (3), is applied to a mixer circuit 49 where it is heterodyned with a portion of the output .omega. from the oscillator 47 to produce a voltage level V(.omega.) proportional to sin.phi..sub.s.
By choosing .eta..apprxeq.1.84, the maximum value of J.sub.1 (.eta.) is obtained, and if the gyroscope output (at the output of amplifier 43) at frequency .omega. is detected, the voltage level at the output of the mixer 49 is EQU V(.omega.)=.alpha..sub.1 I.sub.0 J.sub.1 (.eta.)sin.phi..sub.s( 5)
where V(.omega.) is a voltage amplitude which reflects the sign of .phi..sub.s and for which the sensitivity .DELTA.I/.DELTA..phi. is greatest at small values of .phi..sub.s. There are two types of problems associated with Equation (5).
A first problem is that the sensitivity of the gyroscope decreases as .phi..sub.s increases and actually goes to zero when .phi..sub.s reaches .+-.90.degree.. In other words, because the slope of the sine function (sin .phi..sub.s) goes to zero at .+-.90.degree., small changes in the measured level of V(.omega.) correspond to very large changes in the imputed .phi..sub.s, and thus in the imputed rotation rate. Since the fiber optic gyroscope of FIG. 1 is not reliable near or past a point where the slope of sin .phi..sub.s goes to zero (.+-.90.degree.), the conventional fiber optic gyroscope is limited to a maximum Sagnac phase shift of .+-.90.degree.. This restricts the maximum rotation rate to that value of .OMEGA. which corresponds to .phi..apprxeq.90.degree. and, therefore, limits the overall "dynamic range" of the gyroscope to one-fourth of a fringe.
A second problem is that the output level, V(.omega.), also depends on J.sub.1 (.eta.). Unintentional changes in the level of .eta. will cause J.sub.1 (.eta.) to change and, hence, cause the scale factor of the gyroscope to drift. Changes in .eta. can occur due to amplitude drift in the oscillator 47 that drives the PZT 45. Such changes in .eta. can be controlled by monitoring the level of the output of the oscillator 47. To attain scale factor stability comparable to current sensitivities (in parts per million) requires an amplitude control of 0.1%. Such an output level is not difficult to control. However, the electrical drive amplitude is not the only factor which influences .eta.. Changes in the mechanical properties of the PZT 45 with temperature, changes in how the fiber optic coil 33 loads the PZT 45, creeping in the fiber glass (not shown) on the PZT 45, relaxation of the adhesive bonds (not shown) between the coil 33 and PZT 45, and slowly changing static stresses on the PZT 45 are a few of the possible sources of error that could change .eta., even if the amplitude of the oscillator 47 (which drives the PZT 45) were held perfectly constant. Since some of the above mentioned sources of error can not be modelled, the scale factor stability of the gyroscope of FIG. 1 will be limited to the level of certainty to which all such error sources are identified and stabilized.
One attempt to rectify the above-described problems has been disclosed by K. Bohm, et al. in their article, Direct Rotation-Rate Detection With a Fibre-Optic Gyro By Using Digital Data Processing, 19 Electronics Letters 997-999 (1983). In this article, the first, second and fourth harmonics are detected by means of a 10-step digitization of the output waveform which is used to extract the three amplitudes. The first and second harmonic components are used to contruct an arctangent, while the second and fourth harmonics are used to hold .eta. constant. This technique is an improvement over the usual method because it does take cognizance of the fact that .eta. can change and it increases the dynamic range of the gyroscope by looking at both the sin .phi..sub.s and cos .phi..sub.s components. However, by virtue of the fact that this technique stabilizes .eta. by looking only at cos .phi..sub.s terms, this technique can not develop an accurate error signal when cos .phi..sub.s =O. In addition, the 10-step digitization process is expected to be less accurate, as well as 10 times slower, than a technique in which V(.omega.) or V(2.omega.) are first processed as analog signals and then digitized.